TITLE: GCN CIRCULAR NUMBER: 41345 SUBJECT: EP250812a: NOT, NTT, and GS observations DATE: 25/08/13 15:23:05 GMT FROM: Jonathan Quirola at Radboud University jaquirola1990@gmail.com

A. P. C. van Hoof (Radboud), J. A. Quirola-Vasquez (Radboud), J. N. D. van Dalen (Radboud), D. B. Malesani (DAWN/NBI and Radboud), L. Cotter (UCD), M. Fraser (UCD), G. Corcoran (UCD), P. G. Jonker (Radboud), A. Martin-Carrillo (UCD), A. J. Levan (Radboud and Warwick), F. E. Bauer (SSI and UTA), report on behalf of a larger collaboration:

We observed the field of the fast X-ray transient EP250812a (Zhang et al., GCN 41327, GCN 41339) using the Nordic Optical Telescope (NOT, equipped with the ALFOSC camera), the New Technology Telescope (NTT, with the EFOSC2 instrument), and the Gemini South Telescope (GS, with the FLAMINGOS2, F2, instrument).

Observations with NTT were also carried out in the r filter (3x600 s) starting on 2025-08-13 05:22:07.8 UT, i.e., ~16.2 hr after the trigger. Moreover, NOT and NTT observations were carried out in the z filters (15x80 s and 6x300 s) starting on 2025-08-13 05:31:57.8 and 2025-08-13 05:40:17.2 UT, i.e, ~16.4 and 16.5 hr after the X-ray trigger, respectively. The GS-F2 visited the field using the H filter (20x10 s) starting on 2025-08-13 07:35:11.0 UT, i.e.,~18.4 hr after the trigger.

At the location of the transient reported by Zhang et al. (GCN 41327) and the optical counterpart from SVOM/VT reported by Xin et al. (GCN 41340) and from COLIBRI (Ducoin et al., GCN 41342), also consistent with the EP/FXT (Zhang et al., GCN 41339) and Swift/XRT positions (Evans et al., GCN 41330), no source is detected, down to 3-sigma limiting magnitudes:

H > 22.0 (GS) z > 22.5 (NOT) z > 22.3 (NTT) r > 22.6 (NTT)

These upper limits are in AB magnitudes and were calibrated using nearby stars from the Pan-STARRS and 2MASS catalogues and not corrected by galactic extinction.

We thank the excellent support from the observing staff at Gemini (especially Cinthya Rodrigez), NOT (in particular Emil Knudstrup), and NTT (in particular Pablo Arias).

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